Casey — Notes on the Pleurotomidae. 133 



The European Miocene Iiouaultia Bell., seems to share 

 some of the characters of both Pleuroliria and Gemmula, but 

 has the peripheral carina greatly expanded and crenulate, 

 giving it the general appearance of Oochlesjnr^a, with which 

 it was confounded by Cossmann. It however differs com- 

 pletely in the form and position of the anal sinus, Iiouaultia 

 having the sinus on the expanded peripheral carina, as in the 

 two genera mentioned, while in Cochlesjrira it is situated on 

 the concave f asciolar surface between the periphery and suture . 



Gemmula Weink. 



According to the definition given by Cossmann (Essais de 

 Pal. Comp., 2 Livr., 1896, p. 62) there would seem to be no 

 reason to doubt that P. denticula, the type of Hemipleurotoma 

 Coss., belongs to the same group of species as the living 

 hieneri Doum., monilifera Pease, and several others, in fact 

 the latter is so similar to the American Eocene and Oligocene 

 arnica androtaedens, that, following the views of some authors, 

 it might be considered a mere variety. Although I have not 

 seen gemmata, the type of Gemmula, it is assumed to be en- 

 tirely congeneric with Meneri,fusca, gilchristi and monilifera, 

 which are before me, these four species representing the most 

 typical form of Gemmula in retaining the denticulations of 

 the peripheral carina throughout the growth of the shell. 

 The living alhina Lamk. and deshayesi Desm. are somewhat 

 aberrant in having the denticulation of the peripheral carina 

 confined to the very young or nepionic whorls and gradually 

 becoming lost on the larger whorls, as in the American fossil 

 alternala, of Conrad, and some other species. The genus 

 will prove to be very extensive, retaining all essential charac- 

 teristics of form and sculpture from the very early Eocene to 

 the present time, and there will probably prove to be at least 

 30 species in our Eocene strata alone. The species of Gem- 

 mula vary in size from relatively small to distinctly above the 

 average of the tribe, and resemble Pleurotoma in being devoid 

 of true ribbing or visible lines of growth, in general form, 

 and in the development of spiral carinae, but differ in having 



